MC34280 Datasheet, PDF (14/20 Pages) ON Semiconductor – Power Supply & Management IC for Handheld Electronic Products

MC34280
DETAILED OPERATING DESCRIPTION (Cont’d)
Auxiliary Regulator (Cont’d)
As the Auxiliary Regulator control scheme is the same as
the Main Regulator, equations for conduction mode, Tsw
and Ipk can also be applied, However, h to be used for
caculation is refered to Figure 8, 10, or 12.
If external potentiometer is used for voltage level
adjustment, internal 1.22V reference voltage can be used as
shown in the application diagram of Figure 24.
Current Limit for Both regulators
From Figure 20 and Figure 23, sense devices (senseFET
or senseBJT) are applied to sample coil current as the
low–side switch is ON. With that sample current flowing
through a sense resistor, sense–voltage is developed.
Threshold detector (COMP2 in both figures) detects
whether the sense–voltage is higher than preset level. If it
happens, detector output reset the flip–flop to switch OFF
low–side switch, and the switch can only be ON as next
cycle starts.
Cpor
c = 80n
GND
CVDD
c = 20u
GND
VBAT
GND
Vref GND
Riref
Ren
r = 1000 k
r = 480 k
RLBb
r = 900 k
RLBa
r = 300 k
9
8 765 43 21
10
32
GND
11
31
GND ^PORB
12
30
LOWBAT
13
MC34280
29
LIBATON
14
28
LIBATCL
15
27
16
26
GND
25
GND
17 18 19 20 21 22 23 24
VAUXEN
VBAT
GND
CMAINb
c = 100p
RMAINb
r = 1000 k
1N5817
VBAT
LMAIN
GND
L = 33uH
CMAIN
c = 100u
GND
VAUX
GND
GND ^1N5818
VBAT
LAUX
L = 33uH
Caux
c = 30u
GND
Rauxb
r = 2.2 M
Figure 24. Application Diagram with External Potentiometer for VAUX Adjustment
Rauxa
r = 200 k
GND
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